1. Field of the Invention
The present invention is directed to probe devices for metrology instruments such as atomic force microscopes (AFMs), and more specifically a method of producing a probe devices that allows for precise and repeatable control over cantilever length, tip mass and tip height so as to enable fast scanning AFM operation.
2. Discussion of the Prior Art
Several probe-based metrology instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs), including atomic force microscopes (AFMs), typically characterize the surface of a sample down to atomic dimensions by monitoring the interaction between the sample and a tip on the cantilever probe. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The probe of the typical AFM includes a small cantilever which has a fixed end extending from a base, a sharp probe tip attached to the free end of the lever, generally opposite the base. As discussed further below, the physical properties of the probe greatly impact the scan speed at which the AFM may be operated. In operation, the probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with a deflection detector, such as an optical lever system, an example of which is described in Hansma et al. U.S. Pat. No. RE 34,489. The probe may be scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography, elasticity, or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. Some AFMs can at least selectively operate in an oscillation mode of operation such as TappingMode™ (TappingMode is a trademark of Veeco Instruments, Inc.) operation. In TappingMode™ operation the tip is oscillated, typically at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are particularly important measurement devices in many diverse fields including with particular application in connection with the present preferred embodiments semiconductor manufacturing.
A scanning probe microscope, such as an atomic force microscope (AFM) operates by providing relative scanning movement between a measuring probe and a sample while measuring one or more properties of the sample. A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15. Probe 14 is often a microfabricated cantilever with an integrated tip 17. A scanner 24 generates relative motion between the probe 14 and sample 22 while the probe-sample interaction is measured. In this way images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be an assembly of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY scanner that moves the sample and a separate Z-actuator that moves the probe.
In a common configuration, probe 14 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 14 at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other motion of cantilever 15.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 14 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe.
Often a selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. Note that the sensing light source of apparatus 25 is typically a laser, preferably a visible or infrared laser diode. The sensing light beam can also be generated by other light sources, for example a He—Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. In general, controller 20 generates control signals to maintain a relative constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
A workstation is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM. Notably, scanner 24 often comprises a piezoelectric stack (often referred to herein as a “piezo stack”) or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Additionally, flexures are used to increase the stiffness of actuator in one or more axis, as described in copending application Ser. No. 11/687,304, filed Mar. 16, 2007, entitled “Fast-Scanning SPM Scanner and Method of Operating Same.” Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
At present, the broadening use of SPM has demanded greater performance over a wider range of applications. For example, AFM metrology is increasingly being utilized in semiconductor fabrication facilities, primarily due to recent developments in automated AFM tools able to acquire sample measurements with higher throughput, such as the Dimension® line of AFMs offered by Veeco Instruments Inc. These tools are able to provide a variety of sub-nanoscale measurements, therefore making AFM a viable tool for measuring, for example, “critical dimensions” of device features such as trenches and vias.
No matter the application, a significant limitation to AFM performance is often the speed at which the AFM can scan the sample. As mentioned previously, the construction of the probe device significantly impacts scan speed. Two primary characteristics of probe devices usable for fast scanning applications are a sharp tip and precise control over cantilever dimensions. Techniques are known for producing probes with sharp tips but they are typically low yield and often provide only limited control over the length of the cantilever which ideally is maintained at less than about 50 microns.
More generally, to enable fast scanning in scanning probe microscopy, control must be maintained over the resonant frequency of the cantilever as well as its spring constant, while the damping characteristics of the probe when oscillating must also be considered. These factors are primarily controlled by geometric factors associated with the probes including length of the cantilever, width of the cantilever, cantilever thickness and tip height. As noted, to maintain high yield and performance, precise control of this geometry should be maintained.
In this regard, techniques for producing silicon nitride cantilevers with integral sharp tips are known. For example, in U.S. Pat. No. 5,066,358 to Quate et al. describes a technique for producing silicon nitride probe devices. However, according to the Quate et al. technique, it is difficult to scale to small cantilevers with precise control over the cantilever length, as well as the mass and the height of the silicon tip. More particularly, in processes such as those disclosed in the '358 patent, as well as in U.S. Pat. Nos. 5,021,364 and 5,811,017, electrochemical etch stops must be used in conjunction with a heavily doped silicon or silicon-on-insulator wafers. This is due to the fact that when the required electrochemical wet etch is performed (FIG. 4 of the '358 patent), some structure is required to halt the etch when forming the cantilever (36 in the '358 patent). Doped silicon provides the appropriate structure, thus allowing the silicon to remain intact with the backside etch.
However, there are significant drawbacks to using either heavily doped silicon or silicon-on-insulator wafers. Problems with prior art techniques include high cost, lower production yields, stress bending of cantilevers, and rough backside surfaces that interfere with successful use of the optical lever technique. Many prior art techniques also have issues with errors associated with mask alignment, especially frontside to backside alignment. These issues create a typical manufacturing tolerance for cantilever length and/or tip position of roughly ±5 μm. This manufacturing tolerance makes it difficult to manufacture small cantilevers, for example smaller than 50 μm, or worse smaller than 10 μm, with sufficient accuracy concerning its cantilever spring constant, resonant frequency and tip offset from the free end of the cantilever.
Tip formation is also less than ideal with these known techniques, exhibiting undesirable variations in tip height and tip sharpness. As a result, conventional techniques do not exist that produce short cantilevers through parallel processing on the wafer scale with high yield sharp tips (e.g., tip radius less than 20 nm, for example) and tip heights less than about 4 microns.
As a result, the field of atomic force microscopy was in need of a microfabrication process that produces an AFM probe having an integral tip, and provides precise control over cantilever length to produce probe devices having cantilevers with lengths less than 50 μm, and preferably less than 40 microns and more preferably less than 10 microns.